I. Field of the Invention
This invention relates generally to stent devices of the type intended to be inserted in tubular body organs for maintaining the organ in a patent condition, and more particularly to the design of a tubular stent whose thermoplastic material and geometry allow it to expand by itself from a radially compressed condition to a larger diameter and which can later be resected using an electrosurgical instrument.
II. Discussion of the Prior Art
Various forms of surgical stents are known in the art for maintaining a tubular body organ, such as a vein, artery, bile duct, fallopian tube or urethra, in a patent condition whereby body fluids can continue to flow in a normal fashion. Consider the condition termed benign prostatic hypertrophy where, in the male urinary system, with age, the prostate gland may swell. If the urethra which the gland surrounds is collapsed to the point where the flow or urine from the bladder becomes partially or even fully blocked, surgical intervention is often required. In surgically addressing this problem, a transurethral resection of the prostate is often performed in which portions of the prostate gland are shaved or resected away using an electrosurgical instrument called a resectoscope.
Another approach in treating an enlarged prostate involves inserting a dilatation catheter into the urethra and advancing that catheter until the balloon portion thereof is aligned with the prostate. Then the balloon is inflated to stretch and enlarge the urethra. Another treatment involves the insertion of a stent which functions to re-enforce the urethra at the site so that the tissue involved does not collapse to obstruct urine flow.
Where a stent is to be implanted transurethrally, it is an important characteristic that it possess a low cross-sectional profile to facilitate its being routed to the desired site within the urethra. Once appropriately positioned, it is desirable that the stent expand to a larger diameter and that it remain stable at that diameter over an extended period to provide the necessary support for inhibiting the urethra from again collapsing. Various devices having this property are described in the patent art. For example, in U.S. Pat. No. 4,655,771 to Wallsten, there is described a tubular stent formed from braided metal wire which, when stretched longitudinally, will assume a relatively small diameter, but when it is allowed to spring back to a shorter length, an attendant increase in the diameter takes place. This device suffers from a number of practical problems, not the least of which is the difficulty in properly positioning the stent so that, when released, it will collapse longitudinally and increase in size radially to the point where patency is established along the length of the prostate without having a portion of the stent protrude into the external sphincter so as to result in urinary incontinence or, alternatively, into the bladder where it would serve as a nidus for stone formation.
Moreover, if only a short length stent is called for, there is a tendency for the individual wires comprising the stent to unbraid. Thus, this type of stent may not be usable following a TURP procedure to support only the portion of the urethra at the neck of the bladder. Yet another drawback is that the free ends of the braided wire stent have projecting whiskers that can irritate the lining of the vessel in which it has been implanted.
Also, if a stent of the type described in the Wallsten patent remains in the body for a period of several months, tissue ingrowth occurs and the stent, because of its open construction, becomes incorporated into the vessel wall where it is shielded from the urine. However, should it become necessary to explant the stent for any reason, it becomes extremely difficult to remove it through the urethra.
The Rosenbluth U.S. Pat. No. 4,893,623 describes a tubular stent where the wall of the tube is slit in a predetermined fashion. To implant the stent, it is mounted over a deflated balloon on a dilatation catheter and then routed to the appropriate site in the tubular organ where the stent is to be deployed. The stent is made from a malleable metal so that, when the balloon is inflated, it will stretch the walls of the stent, creating an open lattice pattern. When the balloon is again deflated, the stent will remain stretched to the diameter established by the inflated balloon and the dilatation catheter can again be withdrawn from the body.
The stent arrangement described in the Rosenbluth patent also becomes difficult to remove once tissue ingrowth has occurred. Moreover, it is not self-expanding but, instead, must be stretched to a desired diameter through the application of an outward radial force. When that outside force is removed, the stent does not provide a residual outward radial force against the vessel wall. This may lead to undesired migration of the stent within the hollow vessel subsequent to its implantation and prior to the establishment of tissue ingrowth.